1. Field of the Invention
The present invention relates to an amorphous oxide semiconductor, a thin film transistor using the amorphous oxide semiconductor film, and a method of manufacturing the thin film transistor.
2. Description of the Related Art
Recently, semiconductor devices using metal oxide semiconductor thin films have attracted attention. The thin films are characterized by being able to be formed at low temperatures, and having a large optical band gap so as to be transparent to visible light. Thus, a flexible and transparent thin film transistor (TFT) or the like may be formed on plastic substrates, film substrates, and the like (U.S. Pat. No. 6,727,522).
Conventionally, an oxide semiconductor film used as a TFT active layer is, in order to control the electrical characteristics of the film, generally formed in an atmosphere with oxygen gas introduced thereinto. For example, U.S. Patent Application Publication No. 2007/0194379 discloses a method of forming a thin film transistor (TFT) in which an n-type oxide semiconductor containing indium gallium zinc oxide (In—Ga—Zn—O) is used as a channel layer and indium tin oxide (ITO) is used as source and drain electrodes. In the film forming method described in U.S. Patent Application Publication No. 2007/0194379, by controlling the partial pressure of oxygen in an atmosphere in which the In—Ga—Zn—O film is formed, carrier density is controlled, thereby obtaining a high carrier mobility.
Further, Japanese Patent Application Laid-Open No. 2007-073697 discloses a technology of including water vapor in an atmosphere gas in sputtering as a method of manufacturing a high performance thin film transistor in which the above-mentioned oxide semiconductor film is used as a channel layer. Further, U.S. Patent Application Publication No. 2009/0045397 and Japanese Patent Application Laid-Open No. 2007-194594 disclose technologies of controlling the carrier density by introducing hydrogen into an oxide semiconductor used as a channel layer of a thin film transistor.
However, in a conventional oxide semiconductor film, even when the carrier density is controlled by introducing hydrogen as disclosed in U.S. Patent Application Publication No. 2009/0045397 and Japanese Patent Application Laid-Open No. 2007-194594, the amount of hydrogen in the oxide semiconductor is orders of magnitude greater than the carrier density. In other words, there is a problem that it is impossible to control the carrier density only by the amount of hydrogen in the film. Further, the inventors of the present invention have studied formation of amorphous oxide semiconductors including an amorphous In—Ga—Zn—O-based semiconductor by sputtering to find that the amorphous oxide semiconductors are very prone to take in hydrogen. For example, it has been found that, even when the back pressure of an oxide semiconductor sputtering apparatus is set to 2×10−4 Pa or lower, there is a case in which hydrogen of 1×1020 cm−3 or more is contained. These studies have revealed that it is difficult to form an amorphous oxide semiconductor containing hydrogen of 1×1020 cm−3 or more at room temperature at low cost without pretreatment such as thermal treatment and to make the resistivity thereof appropriate for an amorphous oxide thin film transistor.